This invention relates to an estimation encoding system for efficiently transmitting or accumulating a picture image of variable density, and more particularly to an adaptive quantizer for controlling quantization characteristics according to variations in the input picture image for each of the color components of a color picture.
A variety of encoding band compression systems have been proposed in order to reduce transmission time and cost when a picture image of variable density is transmitted with a narrow bandwidth. A DPCM (differential pulse code modulation) system is well known as one of these systems. The compression effect is further improved, and the degradation in quality of an image due to gradient overload noise can be prevented, by using an adaptive quantization system which is developed from the DPCM system by considering the fact that the human visual characteristic is such that visual quality is lowered for a portion of a picture image where the variation is abrupt but the quality is sufficient for a portion of the picture image where the variation is moderate, so that a quantization characteristic having a coarse quantization level value is used when the input picture element values are large in variation, and a fine quantization characteristic is used when the input picture element values are small in variation; that is, the quantization characteristic is changed adaptively according to the variation of input picture element values.
A method of realizing this feature has been disclosed by Japanese Patent Application No. 38408/1980.
The principle of the method disclosed by Japanese Patent Application No. 38408/1980 will be described briefly. Based on the fact that, locally, a portion of a picture image is similar to those adjacent thereto, the picture image is divided into blocks having a suitable size, and according to the distribution of quantization level values in one block, a quantization characteristic is selected for the next block. Especially in the case where the blocks of a picture image are spatially adjacent to one another, one block is similar to the other blocks surrounding it. Therefore, the probability of selecting an erroneous quantization characteristic for the next block is very small. Furthermore, as the quantization level values of input picture elements are not monitored at all times, but monitored instead for one entire block, a quantization characteristic to be selected is shifted stably. Therefore, degradation of the image can be prevented, and the quantization characteristic can be selected correctly with high probability. Thus, the method has the effect that coding can be achieved with high efficiency.
This invention is intended to apply the adaptative quantizer disclosed by Japanese Patent Application No. 38408/1980 to a color picture, thereby to provide means for encoding a color picture with high efficiency. In the invention, a visual characteristic described below is utilized to selectively employ quantization characteristics. The human eye senses luminance such that it is sensitive to discontinuous variations in luminance as described above, but is insensitive to the magnitude of luminance difference in a region where the luminance changes abruptly (or a high frequency region) and sensitive to that in a region where the luminance changes moderately (or a low frequency region). The sensitivity to color depends on the extent of the area of an object such that as the area of an object is decreased, first blue and yellow sensitivities are lost and, as the area is further decreased, only brightness remains.
In other words, the human eye is sensitive to the variation in luminance in a color picture, insensitive to the magnitude of luminance difference and the color in a local region where the spatial frequency is high, and sensitive to the magnitude of luminance difference and the color in a local region where the spatial frequency is low.